Double wall storage tank

ABSTRACT

A method of making a double wall tank for storing liquids comprises providing an inner tank having substantially cylindrical sidewalls and end walls; wrapping an impervious sheet material around the cylindrical sidewalls; providing a spacer means on exterior surfaces of the end walls; covering the wrapped inner tank and spacer means with reinforced resinous material to form an outer tank; and providing access to inspect an interstitial space between the impervious sheet material and the inner tank.

[0001] This invention is in the field of storage tanks and in particular such tanks with a double wall to contain leaks, and facilitate detection of leaks.

BACKGROUND

[0002] Safety and environmental regulations generally require that tanks for storage of petroleum and like hazardous liquids be double wall tanks. These are essentially an inner tank completely enclosed in an outer tank. During initial testing, in order to detect leaks in the inner or outer tanks, a pressure differential is imposed on the interstitial space between the two tanks. Most commonly a vacuum is imposed and a visual indicator is connected to the interstitial space. If the pressure increases in the interstitial space, as indicated on the visual indicator, a leak is present in one of the inner tanks or outer tanks, requiring repair. To detect leakage from the inner tank after the tank is installed, a tube is generally provided from the top of the tank to the interstitial space at the bottom. Leaks are detected by visually inspecting, or dripping, the bottom of the interstitial space, where leaking liquid from the inner tank will migrate, drawn by gravity.

[0003] In order to satisfactorily detect leaks, fluid must be able to flow to a leak location through the interstitial space. If the leaking fluid is trapped at one location in the interstitial space, that location is then necessarily not in communication with the rest of the interstitial space and so the leak will not be detected.

[0004] For underground installations, these double wall tanks have conventionally been made by covering an inner tank with fibreglass, molten thermoplastic, or similar materials that will resist corrosion from the soil. Conventionally as well, great care is expended to ensure that the inner and outer tanks were spaced apart. U.S. Pat. Nos. 4,640,439 and 5,664,313, both to Palazzo, include a spacing material, such as mesh, in the interstitial space between the two tanks. Similarly in U.S. Pat. No. 4,655,367 to Palazzo, a sheet with projections extending toward the inner tank is wrapped around the inner tank before the outer tank sheath is applied. The projections ensure that there is a space between tanks.

[0005] In U.S. Pat. No. 4,780,946 to Palazzo a plurality of raised beads is provided on the surface of the inner tank ensures the spacing and in U.S. Pat. No. 4,780,947 to Palazzo a resin spacer material is applied to the surface of the inner tank. Similarly in U.S. Pat. No. 4,927,050 to Palazzo, the inner tank is formed with a patterned surface to provide the interstitial space.

[0006] In a somewhat different approach, U.S. Pat. No. 4,744,137 to Palazzo discloses a rigid outer sheath molded with inwardly extending projections. The outer sheath is installed over the inner tank and the projections ensure the spacing.

[0007] In yet another approach, U.S. Pat. No. RE34,203 to Palazzo discloses applying a release agent to the inner tank surface, and then applying a rigid outer sheath of fibreglass or the like, and then breaking the engagement between the inner tank surface and outer sheath by injecting fluid under pressure. The pressure may be as low as 1-5 psi, and sheath movement can as little as 1/1000 of as inch. Similarly in U.S. Pat. Nos. 5,045,263 and 5,129,540, both to Palazzo, an internal lubricating agent is mixed with a curable synthetic resin to form a rigid outer sheath that resists bonding to the inner tank surface. Although not claimed as strictly necessary, preferably in these as well a fluid is injected between the tanks to break any engagement between them.

[0008] Again in U.S. Pat. Nos. 5,816,435 and 6,119,887, both to Palazzo, an expanded selection of release agents is applied over the inner tank surface prior to applying the outer sheath as a resin or extruded molten thermoplastic material. The release agent is stated to depend on the material selected for the outer sheath. For example where the outer sheath is extruded as molten thermoplastic material, the release agent may include a film of generally smooth imperforate material having higher melting point than the temperature of the molten thermoplastic when extruded. As the heated outer sheath is applied over the film, the film wrinkles so as to define a plurality of ridges or grooves generally circumferentially about the tank. The ridges and grooves provide for the free flow of liquids between the inner and outer tanks. Polyester resin film such as MYLAR™ is stated to be suitable.

[0009] U.S. Pat. Nos. 5,060,817 and 5,102,005, both to Trussler, disclose a double wall tank where the outer sheath is formed separately in pieces which are then joined together to enclose the inner tank without bonding to the inner tank. Fluid migration may be enhanced by texturing the adjacent surfaces.

[0010] U.S. Pat. No. 5,752.616 to Watkinson discloses a double wall tank where the interstitial space is provided by a fluid pervious layer, such as open celled foam. U.S. Pat. No. 5,720,404 to Berg et al. discloses a roto-molded double wall tank with inner and outer tanks joined together by ties to create an annular space which can be monitored for leakage. U.S. Pat. No. 5,462,191 to Bart low et al. discloses a double wall tank with ribs and a spacer filament to provide for fluid migration through the interstitial space.

[0011] U.S. Pat. No. 4,869,386 to sharp discloses a double wall tank with ribs and a fibrous material in some portions of the interstitial space, and a separating material such as foam or matting in other portions. In U.S. Pat. No. 5,152,859, also to Sharp a separating agent, such as wax, is applied to the surface of the inner tank to ensure that a subsequently applied fibrous reinforcing material will not adhere to the inner tank or seal closed the annular space between the tanks. The wax can later be heated and removed from the annular space. Other suitable separating agents include grease. As well porous foam or matting can be applied to the inner tank and covered with a Mylar or polyethylene film to prevent the porous material from becoming saturated with a subsequently applied resinous material.

[0012] All of the considerable prior art is very concerned with ensuring that there is a well defined interstitial space between the inner and outer tanks, providing projections, channels, mesh, porous material such as foam, and so forth.

SUMMARY OF THE INVENTION

[0013] It is the object of the present invention to provide a double wall tank that is easily and more economically constructed than prior art tanks, and that includes an interstitial space between inner and outer tanks that allows a free flow of fluid so that leak may be detected.

[0014] The invention provides, in one aspect, a method of making a double wall tank for storing liquids. The method comprises providing an inner tank having substantially cylindrical sidewalls and end walls; wrapping an impervious sheet material around the cylindrical sidewalls; providing a spacer means on exterior surfaces of the end walls; covering the wrapped inner tank and spacer means with reinforced resinous material to form an outer tank; and providing access to inspect an interstitial space between the impervious sheet material and the inner tank.

[0015] In a second aspect the invention provides a double wall tank for storing liquids comprising an inner tank having substantially cylindrical sidewalls and end walls; an impervious sheet material wrapped around the cylindrical sidewalls; a spacer means covering exterior surfaces of the end walls; a reinforced resinous material applied to the impervious sheet material and spacer means to form an outer tank; and an access port operatively connected to an interstitial space between the impervious sheet material and the inner tank to enable leak detection.

[0016] The present inventors have found that essentially any film of impermeable material may be wrapped around the inner tank, and the outer sheath of fiberglass or the like simply applied over top in the conventional manner. For example MYLAR™ or a plastic film such as the polyethylene film commonly used for vapor barrier may be used. The film does not need to be wrinkled by heat, or include bumps or projections. The interstitial space, though small, is larger than the molecular size of a stored liquid, allowing migration of the liquid as required for leak detection. An interstitial space on the end walls, communicating with the interstitial space between the film and the cylindrical wall, is provided by any of the conventional spacer means.

DESCRIPTION OF THE DRAWINGS

[0017] While the invention is claimed in the concluding portions hereof, preferred embodiments are provided in the accompanying detailed description which may be best understood in conjunction with the accompanying diagrams where like parts in each of the several diagrams are labeled with like numbers, and where:

[0018]FIG. 1 is a perspective view of an inner tank of the invention wrapped with impervious sheet material;

[0019]FIG. 2 is a cross-section view of the wall of the completed double wall tank;

[0020]FIG. 3 is a side view of a completed double wall tank;

[0021]FIG. 4 is a cross-section view of the access port where same pierces the inner tank wall at the bottom of the tank.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0022] A method of making a double wall tank 10 for storing liquids is disclosed. The method comprises providing an inner tank I having substantially cylindrical sidewalls 2 and transverse end walls 3. An impervious sheet material 4 is wrapped around the cylindrical sidewalls 2 with an overlap 7 as illustrated in FIG. 1 in order to prevent the subsequent layer of fiber glass or similar material from bonding to the inner tank 1. The impervious sheet material 4 can be MYLAR™, or a plastic film such as the polyethylene film commonly used for vapor barrier.

[0023] A conventional spacer means 5, such as mesh protected by a film, wax, or the like is provided on exterior surfaces of the end walls 3. The wrapped inner tank 1 and spacer means 5 is covered with reinforced resinous material, such as fiberglass, to form an outer tank 6. This spacing on the end walls 3 and formation of the outer tank 6 are well known in the prior art.

[0024] No attempt is made, as is emphasized in the prior art, to ensure that there is a measurable interstitial space between the inner tank 1 and the impervious sheet material 4. The interstitial space that results using the method is very small, however will readily allow liquid to migrate, and gather at the bottom of the interstitial space

[0025] For a typical underground installation, access to inspect the interstitial space between the impervious sheet material 4 and the inner tank 1 is conveniently provided by an access port 8 which extends through the inner tank 1. The access port 8 pierces the wall of the inner tank 1 at the bottom end thereof, and is covered by the impervious sheet material 4 as illustrated in FIG. 4. Leaking liquid thus migrates to the bottom of the tank 10 and can be detected by visually checking the port 8, or by dipping or other detection means. Other locations and detection means could be used depending on the installation, or other conditions of use.

[0026] The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous changes and modifications will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all such suitable changes or modifications in structure or operation which may be resorted to are intended to fall within the scope of the claimed invention. 

I claim:
 1. A method of making a double wall tank for storing liquids, the method comprising: providing an inner tank having substantially cylindrical sidewalls and transverse end walls; wrapping an impervious sheet material around the cylindrical sidewalls; providing a spacer means on exterior surfaces of the end walls; covering the wrapped inner tank and spacer means with reinforced resinous material to form an outer tank; and providing access to inspect an interstitial space between the impervious sheet material and the inner tank.
 2. The method of claim 1 wherein the reinforced resinous material comprises fiberglass.
 3. The method of claim 1 wherein the impervious sheet material comprises a plastic film.
 4. The method of claim 1 wherein the impervious sheet material comprises MYLAR™ film.
 5. The method of claim 1 wherein the impervious sheet material comprises polyethylene sheeting.
 6. A double wall tank for storing liquids comprising an inner tank having substantially cylindrical sidewalls and transverse end walls; an impervious sheet material wrapped around the cylindrical sidewalls; a spacer means covering exterior surfaces of the end walls; a reinforced resinous material applied to the impervious sheet material and spacer means to form an outer tank; and an access port operatively connected to an interstitial space between the impervious sheet material and the inner tank to enable leak detection.
 7. The tank of claim 6 wherein the reinforced resinous material comprises fiberglass.
 8. The tank of claim 6 wherein the impervious sheet material comprises a plastic film.
 9. The tank of one of claim 6 wherein the impervious sheet material comprises MYLAR™ film.
 10. The tank of claim 6 wherein the impervious sheet material comprises polyethylene sheeting. 